Field of the Invention
In the process of forming multilayer printed circuit boards, it is known that the bond of the prepreg substrate to the copper for the internal layers is improved when an oxide is provided on the copper surface prior to lamination. When making printed circuit boards using the substractive process, wherein an electrodeposited foil copper is first laminated to the substrate, this oxide on the copper surface is provided prior to the lamination of the copper foil to the substrate. Then, after the etching of the printed circuit lines using conventional substractive processes, the remaining copper surface is already oxidized and is easily bonded to the additional prepreg layer of the printed circuit board.
However, when additive copper is used, these bonds cannot be as easily achieved, since the wetting process of the additive copper plating prior to the application of an oxide opens up the surface of the additive copper. This tends to weaken the bond between the oxide and the copper such that when lamination occurs at elevated temperatures, the oxide-to-copper bond breaks, causing very low bond strengths. This problem becomes much more critical as the internal circuit lines become thinner and finer, resulting in circuit discontinuities and other attendant problems.